The present invention relates to a particle separator for removing particles from an exhaust gas stream of an internal combustion engine. The present invention also relates to a method for removing particles from an exhaust gas stream of an internal combustion engine.
During combustion in a diesel engine, and also during direct injection with a spark ignition engine, emissions of particles occur that among other reasons result due to the incomplete combustion of the fuel, and predominantly comprise carbon black. To burn this carbon black, GB 895,990 A proposes burning or treating them with NO2 as oxidizing agent instead of oxygen.
It is furthermore known from U.S. Pat. No. 4,902,487 to separate off carbon particles at a filter, and to subsequently oxidize the particles with NO2. To produce the NO2, pursuant to DE 28 320 02, for example, the NO contained in the exhaust gas stream of the internal combustion engine can be oxidized on a platinum-containing catalyst to form NO2. The proportion of NO2 in relationship to the total NOx is thereby significantly increased, whereby this reaction occurs starting at approximately 180° C. In contrast, the combustion of the particles by NO2 begins at temperatures starting at 250° C. A particle oxidation in the gas phase practically does not occur. For this purpose, a deposition on a solid material is necessary in order to achieve an adequate retention time, which is generally realized by the use of a particle filter.
A significant drawback of such a procedure, however, is the high exhaust gas counter pressure that is caused by the filter. This becomes noticeable by an increased fuel consumption. In addition, during the operation non-combustible constituents are also deposited on the filter, such as lubricating oil ash. As a result, the exhaust gas counter pressure continuously increases, so that the filter must be regularly and frequently replaced in order to avoid an even greater fuel consumption, and possibly also damage to the internal combustion engine. Furthermore, insufficient oxidation can result in an ever increasing coating with soot, which can ultimately lead to clogging of the filter.
To avoid these drawbacks, it is already know from EP 1 072 765 B1 to use a particle separator that has no filter, whereby the exhaust gas stream is guided along a structural surface and is constantly deflected, so that the particles can separate out from the exhaust gas stream by thermophoresis, convection or diffusion. With particle separation by thermophoresis, the surfaces of the particle separator are cooled, so that the surfaces are significantly cooler than is the exhaust gas stream. As a result, the particles are deposited or precipitate on the surface, where they are catalytically oxidized with the NO2 produced at an oxidation catalytic converter. With a particle separator designed according to the convection principle, this surface structure is configured such that the particles are constantly forced toward surface contact, and are then separated off at these surface structures, where they can finally also be catalytically oxidized with the aid of the NO2. Particle separators designed pursuant to the diffusion principle are provided with so-called flow dead zones, for example on the lee side of guide plates. In these flow dead zones, the flow velocity decreases toward zero, so that here the exhaust gas stream has a relatively long retention time, so that particles can diffuse out of the exhaust gas stream into the stream dead zones in order to compensate for the concentration differences that locally exist here. However, a drawback of this is that the separating off of the particles by diffusion deteriorates as the particle diameters increase, so that here larger particles cannot be separated off at all or only very slightly. Since with the adoption of spherical particles the particle mass increases pursuant to mParticle=⅙×d3×π×ρ, with the particle diameter d to the third power, with these separators it is possible to achieve only a relatively low reduction of the particle mass; in other words, essentially only very fine particles are removed, which means that as previously, a large residual particle mass remains in the exhaust gas stream, which constitutes the greater particle mass.
In contrast, it is an object of the present invention to provide a particle separator, as well as a method, for removing particles from an exhaust gas stream of an internal combustion engine, by means of which all sizes of particles, in other words in particular very fine particles as well as larger and/or heavier coarse particles, can reliably and in a straightforward manner be removed from the exhaust gas stream.